Vacuum diffusion pump



June 1969 H. R. SMITH, JR

VACUUM DIFFUSION PUMP Filed Nov. 8, 1967 INVENTOR.

HUGH R. SMITH JR. MA "364w,

ATTORNEYS United States Patent US. Cl. 230101 4 Claims ABSTRACT OF THE DISCLOSURE A slit type vacuum diffusion pump is described wherein an elongated central structure is disposed in an elongated outer casing in such a manner that the distance of vapor jet travel between the central structure and the outer casing is constant across a space completely surrounding the central structure.

This invention relates to vacuum diffusion pumps and, more particularly, to a vacuum diffusion pump of high efficiency and which overcomes certain size limitations which are encountered by previously known vacuum diffusion pump designs.

Vacuum diffusion pumps normally include a plurality of nozzles which establish a sheet or stream of oil vapor at an angle across a pumping duct to a condenser. Molecule-s of gas entering the pumping duct are entrained in the stream and are carried to a suitable discharge duct on the opposite side of the stream from the space being evacuated. The oil is collected and recirculated after it condenses.

Many vacuum diffusion pumps have heretofore had pumping ducts of annular configuration and a centrally disposed or encircling array or arrays of nozzles. One satisfactory form of such a pump is shown and described in US. Patent No. 3,203,624. Cylindrical type vacuum diffusion pumps have been used successfully for many purposes. Nevertheless, certain applications requiring high pumping capacity may require more than one cylindrical type pump, since there are limitatons on the maximum diameter with which pumps of this type can be made. This limitation exists because there are practical limitations on the pressure which is attainable in the oil vapor in order that the vapor stream have a suitable trajectory and sufficient density all the way from the nozzles to the condenser wall. The necd for more than one pump may result in excessive system size or cost.

So-called slit pumps, as hereinafter described, do not have the aforementioned size limitation and very large pumps of the slit type have been built. Previously known designs of slit pumps have included a pumping duct or ducts having an elongated rectangular cross section and a nozzle arrangement for establishing a sheet of vapor across the duct. Such pump designs are shown and describe din U.S. Patents 3,101,169 and 3,203,624. Although satisfactory for many purposes, known designs for slit pumps have produced interactions at the end of the pumps. Accompanying pumping losses may have a serious effect on efficiency with respect to very large capacity pumps.

It is an object of this invention to provide an improved vacuum diffusion pump.

Another object of the invention is to provide a vacuum 3,450,336 Patented June 17, 1969 diffusion pump of high efficiency and which overcomes certain size limitations which are present in previously known diffusion pumps.

A further object of the invention is to provide a slit type vacuum diffusion pump of very large capacity and which avoids excessive pumping losses.

Other objects of the invention will become apparent to those skilled inthe art from the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is an end view, with part broken out, Of a vacuum diffusion pump constructed in accordance with the invention; and

FIG. 2 is a sectional view taken along the line 2-2 of FIGURE 1.

Very generally, the vacuum diffusion pump of the invention comprises condenser means 11 and vapor stream producing means 12 which definea duct 13 having a pair of generally parallel elongated side sections and a pair of generally arcuate end sections joining the ends of the side sections. The condenser means and the vapor stream producing means are of uniform spacing and complementary configurations in at least one region of the duct. The vapor stream producing means are directed for establishing a sheet of vapor of substantially uniform width which extends across the duct around the full extent of the cross section thereof at said region.

Referring now more particularly to the drawings, the vacuum diffusion pump of the invention will be described in greater detail. The condenser means 11 comprise a tubular casing 14 provided with an open top end fitted with a flange 16 for connection to a high vacuum chamber, not illustrated, which is to be evacuated. The casing 14 consists of two generally parallel elongated side walls 17 and 18 joined at their ends by semi-cylindrical end walls 19 and 21. The casing is cooled by coolant conducting tubes 22 around its outside.

The vapor stream producing means 12 occupy the central portion of the space defined by the casing 14 and define, with the casing, the duct 13. The vapor stream producing means 12 in the illustrated embodiment comprise a central structure closed at the top and having openings 23 at the bottom. At each of various levels or regions 25, 26, 27 and 28 along the central structure 12, a surrounding array of vapor nozzles 29 is provided. The nozzles aim outwardly and downwardly at a suitable angle for entraining gas molecules as described below. The nozzles at each region are spaced equidistant from the outer casing 14 along the direction of the vapor stream, indicated by the arrows 30 at each region. Accordingly, the step-shaped duct 13 is comprised of a pair of generally parallel elongated side sections (between the central structure 12 and the side walls 17 and 18), and a pair of generally arcuate end sections between the central structure 12 and the end Walls 19 and 21) joining the ends of the side sections.

Oil is contained in a pool 24 at the bottom of the central structure 12, and heat for vaporizing the oil in the pool is supplied by suitable heating means, such as a gas or electric heater 31. Pipes or tubes 32, through which cooling water may be circulated when desired, may be provided within the structure adjacent to the wall heated by the heater to cool the oil pool 24 when the pump is to be shut down, thereby facilitating rapid shut down.

The oil vapor produced by the heating apparatus passes upward inside the central structure 12, the direction of vapor flow being indicated in FIGURE 2 of the drawing by the arrow 33. At the various levels or regions 25-28 along the central structure, the vapor issues from the nozzles 29. The nozzles in each array are arranged so that the vapor issuing therefrom forms a sheet of vapor in which the vapor density and velocity are substantially uniform. The direction of vapor flow is downwardly and outwardly as indicated by the arrows 30 in both FIGURES 1 and 2.

Molecules from the high vacuum chamber connected to the top open end of the pump casing 14 randomly enter the elongated annular duct 13 between the outer casing 14 and the central structure 12. Gas molecules entering the paths of the above-mentioned vapor jets are entrained and carried downward, eventually being expelled through a pipe 36 at the lower end of the casing. The pipe 36 may be connected to mechanical pumps for maintaining a partial vacuum on the outlet side of the diffusion pump, as is common in the art.

The walls of the outer casing or condenser 11, as previously mentioned, are cooled by cooling pipes 22 attached to the outer surface of the casing and through which a continuous flow of cooling water or the like may be passed. The vapor stream strikes the cooled inner surface of the outer casing and the oil vapor condenses on the relatively cold walls. The condensed oil runs down the sides of the casing and is collected in a pool 37 at the bottom. From the collection pool 37 the oil eventually returns to the vaporization pool 24 through the openings 23 in the central structure to be reheated into vapor. It may be seen, therefore, that the oil is continuously circulated within the vacuum pump, whereas the non-condensible gas molecules entrained by the vapor are expelled through the pipe 36.

One way in which the capacity of a vacuum diffusion pump may theoretically be increased is to enlarge the space between the vapor stream producing means and the condenser. Because there are practical limitations on the pressure which is obtainable in the oil vapor, there is generally a limitation on how wide the space between the vapor stream producing means and the condenser may be made. When this space becomes too great, the vapor will not have a sufficiently straight trajectory and density all the way to the condenser wall to produce a satisfactory pumping action.

In order to produce larger capacity pumps, the so-called slit pump has been proposed. As mentioned above, previously known designs have included an elongated rectangular duct, and nozzles for establishing an appropriately directed sheet of vapor. Such pumps have, theoretically, no size limit, since the rectangular duct may be elongated as needed. Interactions between the vapor sheet and the walls at the ends of the rectangular pumps, however, may have a serious effect on efficiency, due to accompanying pumping losses.

In the pump of the invention, the outer casing 14 and each array of nozzles 29 are of uniform spacing and complementary configurations at the respective levels 25-28. Such is true even at the ends of the slit or pumping opening so that the nozzles may establish a sheet of vapor of substantially uniform width which extends across the duct 13 between the central structure 12 and the outer casing 14. Each uniform sheet is present around the full extent of the cross section of the duct 13 so that pump ing losses are minimized.

By constructing a pump in the foregoing described configuration, size limitations are obviated and the pump may be made of any reasonable capacity desired. In designing a pump according to the invention, the spacing and hence the vapor sheet width may first be selected for optimum operational characteristics depending upon pressure, heat losses, etc. Then the capacity required may be attained by selection of an appropriate slit length, i.e., selection of the longer cross sectional dimension of the 4 outer casing 14. Besides the circular configuration a the ends of the pump, alternatives may be found satisfactory. For example, short linear segments or parabolic curves may be utilized providing that the vapor stream path length from the nozzles 29 in each array to the condenser is the same or substantially the same at all positions. This insures that a uniform flow of entraining vapor is maintained and insures that there are no gaps in the vapor sheets through which gas molecules may escape back into the vacuum enclosure. The term generally arcuate, as used herein, is intended to include such modifications.

As a further alternative, the nozzles at each level may be arranged to direct the vapor inwardly and downwardly from an outer or surrounding vapor stream producing means to condense on a cooled central structure.

Although the illustrated pump is shown having a vertical axis, it may be possible, through suitable modification of the means for collecting and heating the oil, to dispose the pump on a horizontal axis. This may facilitate evacuation of certain furnace structures wherein it is desirable to place other items directly beneath the vacuum enclosure.

It may therefore be seen that the invention provides an improved vacuum diffusion pump of the slit type wherein efiiciency is maintainable for large pumping capacities. The configuration of the pump is such that harmful vapor stream-wall interactions at the ends of the pump with accompanying pumping losses are eliminated.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such other modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A vacuum diffusion pump comprising a vapor stream producing means and a vapor condenser means having mutually adjacent walls defining therebetween an elongated duct, said duct including a pair of generally parallel elongated side sections and a pair of generally arcuate end sections joining adjacent ends of said side sections, said duct having a major longitudinal axis lying generally in a single plane, said vapor stream producing means defining a plurality of nozzles disposed in a plane parallel to the plane of the said major longitudinal axis of said duct, each of said nozzles being adapted to direct a stream of vapor toward a wall of said vapor condensing means for impingement thereon, said nozzles being uniformly spaced from the wall of said vapor condensing means upon which vapor emitted from said nozzles impinges, said vapor stream producing means establishing sheets of vapor of substantially uniform width extending across said duct from said nozzles toward said vapor condensing means.

2. A vacuum diffusion pump in accordance with claim 1, wherein said vapor stream producing means defines a plurality of said nozzles disposed in each of a plurality of spaced planes parallel to the plane of the said major longitudinal axis of said duct, wherein each of said nozzles of each of said planes is adapted to direct a stream of vapor toward a wall of said vapor condensing means for impingement thereon, and wherein the said nozzles of each of said planes are uniformly spaced from the wall of said vapor condensing means upon which vapor emitted from said nozzles impinges.

3. A vacuum diffusion pump according to claim 1 wherein said condenser means include an outer casing having an inner surface comprised of a pair of generally parallel elongated side walls and a pair of generally semicylindrical end walls joining said side Walls, and wherein said vapor stream producing means include a central structure disposed within said outer casing, and spaced therefrom, said central structure having a surrounding array of vapor nozzles at said region spaced equidistant from said outer casing along the direction of vapor flow.

4. A vacuum difiusion pump comprising, a tubular outer casing having a pair of generally parallel elongated side walls and a pair of semi-cylindrical end Walls joining said side Walls, means for cooling said outer casing, a central structure disposed in said outer casing and spaced therefrom, said central structure having a plurality of nozzle means for establishing a corresponding plurality of sheets of vapor extending from said central structure to said outer casing, across said duct, said nozzle means being spaced from each other along said duct, each of said nozzle means completely surrounding said central structure and being of uniform spacing from said outer casing along the direction of vapor flow to provide a substantially uniform width sheet of vapor completely surrounding said central structure between said nozzle means and said outer casing across said duct.

References Cited 5 UNITED STATES PATENTS 2,691,431 10/1954 Simpson 230 101 2,394,630 7/1959 Hiesinger 230-101 3,101,169 8/1963 Smith 230-101 10 3,203,624 8/1965 Smith: 230 101 DONLEY J. STOCKING, Primary Examiner. WARREN J. KRAUSS, Assistant Examiner. 

